wchar_t is defined in /usr/include/stddef.h on Solaris

--- include/X11/Xlib.h
+++ include/X11/Xlib.h
@@ -71,7 +71,7 @@
 #endif
 #endif
 
-#ifndef X_WCHAR
+#if !defined(X_WCHAR) || defined(__sun__)
 #include <stddef.h>
 #else
 #ifdef __UNIXOS2__